This study's objectives are to determine maternal blood progesterone (P4) concentrations during normal baboon gestation, and the MCR and blood production rate of P4 during baboon pregnancy. Serum P4 was estimated by radio immunoassay using an antiserum raised to conjugated 11B-hydroxy progesterone in rabbits. P4 MCR was estimated from the serum levels of H3-P4 during a constant infusion of H3-P for 90-185 minutes.